Method and apparatus for electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen

ABSTRACT

A working electrode for an electrochemical reactor, the electrochemical reactor comprising a working electrode, a counter electrode, and an ion-selective electrolyte; the working electrode comprising an electric conductive ceramic oxide material having the general formula: A 2 A′ (1−x) B y B′ (1−y) O (3−Δ)  wherein A and A′ designate first substitution metals of similar sizes, said first substitution metals having a high efficiency for reducing vacancies for oxygen ions, 0≦x≦1; B and B′ designate second substitution metals of similar sizes, said second substitution metals being of smaller sizes, said second substitution metals being of smaller sizes than those of said first substitution metals, and having a high transition efficiency between oxidation states, 0≦y≦1; O designates oxygen; and Δ is a small number, positive or negative, that allows for compensation of differences in valences of said metals. An electrochemical reactor comprising said working electrode. Methods and an electrochemical reactor for reduction of nitrogen oxides in a mixture og nitrogen oxides and oxygen, the electrochemical reactor comprising a working electrode, a counter electrode, an ion-selective electrolyte, and a nitrogen absorber for absorbing nitrogen oxides; wherein said nitrogen absorber is adapted for electrochemical regeneration thereof.

1. BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus forelectrochemical reduction of nitrogen oxides in a mixture of nitrogenoxides and oxygen.

In an aspect, the invention relates to a working electrode for anelectrochemical reactor, an electro-chemical reactor comprising such aworking electrode, a method of reducing nitrogen oxides in a mixture ofnitrogen oxides and oxygen using an working electrode comprising anelectric conductive ceramic of lanthanum manganite doped with an oxygenion vacancy quencher.

In another aspect, the invention relates to an electro-chemical reactorcomprising nitrogen oxide absorber adapted for electrochemicalregeneration, and a method of electrochemical reduction of nitrogenoxides in a mixture of nitrogen oxides and oxygen using such anelectro-chemical reactor.

THE TECHNICAL FIELD

In the present context the expression “nitrogen oxides”, which are oftendenoted by the term NO_(x), is intended to designate one or morecompounds of oxygen and nitrogen, e.g. NO and NO₂, etc. Further, theexpression “nitrogen oxide absorber” is intended to designate anabsorber for nitrogen oxides, e.g. in form of a compound or acomposition of compounds.

Reduction of NO_(x) in presence of oxygen is known.

In a method adapted to combustion processes, an excess of fuel is addedfor a short period of time thereby providing a reducing agent, i.e.addition of CH, whereby NO_(x) is reduced according to the concurrentreactions:2NO_(x) +xCH+x/2O₂->N₂ +xCO₂ +x/2H₂O  (1)11/2O₂+CH->CO₂+^(1/2)H₂O  (2)

However, the addition of CH affects the combustion processes and therebythe produced heat of the engine.

In electrochemical reduction of NO_(x) in presence of O₂, concurrentelectrode processes between electrons and NO_(x) and O₂ takes place atthe working electrode, e.g. as expressed by the electrode processes atthe cathode:2NO_(x)+4xe⁻->N₂+2xO²⁻  (3)O₂+4e⁻->2O²⁻  (4)

For a given potential and current density, the available electrons reactwith either of the reactants NO_(x) or O₂

A method of increasing the selectivity of NO_(x) -reduction relative toO₂-reduction comprises increasing the amount of NO_(x) relative to thatof O₂ prior to electrochemical reduction. Alcaline earth metals such asMgO or CaO have been used to absorb NO_(x). Subsequently, NO_(x) isreleased by heat regeneration before electrochemical reduction ofNO_(x).

Another method of increasing the selectivity of NO_(x)-reductionrelative to O₂-reduction comprises increasing the access of NO_(x) toreactive electrons of the working electrode compared to the access ofO₂, or equivalent by increasing access of electrons of the workingelectrode to NO_(x) compared to access of electrons to O.

PRIOR ART DISCLOSURES

U.S. Pat. No. 5,022,975 discloses a solid state electro-chemicalpollution control device for altering the composition of a gas streamincluding removing SO and NO; in an embodiment said device comprisesgadolinia stabilized ceria as electrolyte.

U.S. Pat. No. 5,401,372 discloses an electrochemical catalytic reductioncell for reduction of NO_(x) in an O₂-containing exhaust emission usinga gas-diffusion catalysts such as supported vanadium oxides with anelectron collecting layer such as a conductive perovskite-type oxide,e.g. LSM.

U.S. Pat. No. 5,456,807 discloses a method and apparatus for selectivelyremoving nitrogen oxides from gaseous mixtures comprising absorption ofNO_(x) with NO_(x) adsorbents, heating release of absorbed NO_(x) andelectrochemical reduction of NO_(x) to N₂ and O₂ in solid-oxidelectrochemical cells.

WO 97/44126 discloses an electrochemical reactor comprising a mixedion-selective electrolyte and electrode material of heat treatedgadoliniumoxide doped with 20% CeO and containing about 6 vol.-%lanthanium oxide doped with 20% strontiumoxide for reduction of carbonblack in nitrogen containing 20% oxygen. Nothing is indicated norsuggested about reducing NO_(x) to N₂ and

2. DISCLOSURE OF THE INVENTION OBJECT OF THE INVENTION

It is an object of the present invention to seek to provide an improvedmethod and apparatus for selective electrochemical reduction of nitrogenoxides in presence of oxygen.

It is an object of the present invention to seek to provide such animproved method and apparatus for selective electrochemical reduction ofnitrogen oxides in presence of oxygen in gaseous combustion mixtures.

Further objects appear from the description elsewhere.

Solution According to the Invention

According to the present invention, these objects are fulfilled byproviding a working electrode for an electrochemical reactor, theelectrochemical reactor comprising a working electrode, a counterelectrode, and an ion-selective electrolyte; the working electrodecomprising an electric conductive ceramic oxide material having thegeneral formula:A_(x)A′_((1−x))B_(y)B′_((1−y))O_((3−δ))

-   -   wherein A and A′ designate first substitution metals of similar        sizes, said first substitution metals having a high efficiency        for reducing vacancies for oxygen ions, 0≦x≦51;    -   B and B′ designate second substitution metals of similar sizes,        said second substitution metals being of smaller sizes than        those of said first substitution metals, and having a high        transition efficiency between oxidation states, 0≦y≦1;    -   O designates oxygen;    -   and δ is a small number, positive or negative, that allows for        compensation of differences in valences of said metals.

It has surprisingly turned out that selecting a working electrodecomprising an electric conductive ceramic oxide material having the ABO₃formula as defined, the number of vacances in the oxygen ion lattice canbe minimized whereby oxygen ion conductivity in the ceramic oxidematerial can be minimized.

Consequently, a working electrode having high selectivity for reductionof NO_(x) and at the same low activity for reduction of O₂ can beprovided.

The components A, A′, B, and B′ of the AA′BB′0₃ material can be selectedwithin wide ranges.

In a preferred embodiment, the working electrode A is selected from thegroup consisting of rare earth metals: Sc, Y, La, Ce, Pr, Nd, Sm, Gd,Tb, Dy, Ho, Er, Tm, hb, Lu; metals of group 3 a: Al, Ga, and In; andcroup 3 b: Sc, Y, La of the periodic table, preferably La, Gd, In and Y;

and A′ is selected from the group consisting of alkaline earth metals:Mg, Ca, Sr, and Ba; and Eu, preferably Ca, Sr, Ba, and Eu

whereby it is achieved that the electrical and chemical/catalyticproperties of the working electrode can be tailored within wide ranges.

In another preferred imbodiment, B and B′ are selected from the groupconsisting of transition metals:

-   croup 1 b: Cu and Ag;-   group 2 b: Zn;-   group 3 a: Ga, In, and Tl;-   group 3 b: Sc, and Y;-   group 4 b: Ti, Zr, Hf;-   group 5 b: V, Nb, Ta;-   group 6 b: Cr, Mo, W;-   group 7 b: Mn and Re; and-   group 8: Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt;    preferably Cr, Mn, Fe, Co, and Ni    whereby it is further achieved that the electrical and    chemical/catalytic properties of the working electrode can be    tailored within wide ranges.

The actual elements and stoichiometric coefficients can be selected byexperimentation.

For y=0, a particularly preferred working electrode comprises a LSMmaterial.

In a preferred embodiment, the ceramic oxide comprises lanthanummanganite doped with strontium oxide, La_(x)Sr_(1−x)MnO₃, thestoichiometric coefficient 1−x being in the range 0.05 to 0.20,preferably 0.10 to 0.18, most preferred about 0.15 whereby a goodselectivity can be obtained for reduction of nitrogen oxides compared toreduction of oxygen.

An aspect of the invention relates to an electrochemical reactorcomprising the working electrode, the counter electrode, and theion-selective electrolyte wherein the working electrode is according tothe invention. Such a reactor can be utilised for the reduction ofnitrogen oxides in the exhaust gas from diesel engines or lean burn ottoengines, where the high content of oxygen precludes the use of standardtechniques, such as chemical reduction in a three way catalyst, for thereduction of the content of nitrogen oxides.

Another aspect of the invention relates to a method of reducing nitrogenoxides in a mixture of nitrogen oxides and oxygen, the methodcomprising: providing an electrochemical reactor comprising a workingelectrode, a counter electrode, and an ion-selective electrolyte; saidworking electrode being adapted to reduce nitrogen oxides to nitrogenand oxygen, and said working electrode being adapted to suppressreduction of oxygen to oxygen ions; said working electrode processesbeing substantially according to the cathode electrode processes:2NO_(X) +2xe⁻->N₂+2x 0 ²⁻  (a)O₂+4e⁻->2O²⁻  (b)and the anode electrode process:2O²⁻->O₂+4e⁻  (c)said cathode electrode processes (a) and (b) being carried out at apotential selected within a range of −1500 mV to +1500 mV between saidworking electrode and said counter, electrode, and at a temperaturewithin a range of 200 to 500° C.;and said working electrode comprising an electric conductive ceramic oflanthanum manganite; said lanthanum manganite being doped with an oxygenion vacancy quencher for quenching vacancies for oxygen ions; saidoxygen ion vacancy quencher being in an effective amount to suppresssaid reduction of oxygen to oxygen ions at the working electrode so thatthe rate of reduction of nitrogen oxides is faster than the rate ofreduction of oxygen at the selected potential and temperature.

In a preferred embodiment said selected potential is selected within therange from −200 mV to 800 mV, said potential being measured versus ahydrogen electrode of 8% H₂O and 3% H₂ in Ar whereby it is obtained thatthe selectivity is further enhanced, and the total power demanddecreased by lowering the potential as much as possible, withoutreaching a situation where the reduction rate becomes too small.

In a particularly preferred embodiment said oxygen ion vacancy quencheris selected from the group consisting of Sr, Ca, Ba, and Eu.

In still another embodiment said method uses electrochemical reactorcomprising a working electrode according to the invention wherebyparticular improved selectivity of reduction of nitrogen oxides isobtained.

In some applications the concentration of nitrogen oxides is low.Consequently, a preconcentration of nitrogen oxides may be desired.

An aspect of the invention relates to an electro-chemical reactor forreduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen,the electro-chemical reactor comprising a working electrode, a counterelectrode, an ion-selective electrolyte, and a nitrogen absorber forabsorbing nitrogen oxides; wherein said nitrogen absorber is adapted forelectrochemical regeneration thereof; whereby it is achieved thatnitrogen oxides can be adsorbed readily, even from gas mixtures with lowconcentrations of nitrogen oxides. Said electrochemical reactor can theneasily regenerate the NOx adsorber by electrochemical reduction, withoutthe need for addition of external heat or a chemical reducing agent.

In a preferred embodiment said nitrogen absorber and said workingelectrode are intermixed whereby it is achieved that there is anintimate contact between the adsorbed NOx-containing species and theworking electrode. This assures a fast, selective and efficientreduction of the NOx-containing compound.

In another preferred embodiment said nitrogen absorber comprises aporous layer on said working electrode whereby a separate absorber isobtained which can be advantageous for some applications with respect toeasy assembling and maintenance.

In a preferred embodiement the nitrogen absorber comprises a material ora combination of materials selected from the group consisting of Na₂O,K₂O, MgO, CaO, SrO, and BaO, preferably BaO whereby nitrates andnitrites are easily formed in the presence of nitrogen oxides. Further,these nitrates and nitrites can easily be converted back to oxides underreducing conditions at elevated temperature.

In a preferred embodiment of this electrochemical reactor said workingelectrode is a working electrode according to the invention.

In another aspect the invention relates to a method of electrochemicalreduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen,the method comprising:

-   providing an electrochemical reactor comprising a working electrode,    a counter electrode, an ion-selective electrolyte, and a nitrogen    oxide absorber for absorbing nitrogen oxides;-   absorbing nitrogen oxides from the mixture of nitrogen oxides and    oxygen into said nitrogen oxide absorber;-   electrochemically regenerating said nitrogen oxide absorber by    electrochemically reducing species containing said absorbed nitrogen    oxides; said species being produced during said absorption.

In a preferred embodiment the nitrogen oxides are absorbed in saidnitrogen oxide absorber without applying and the counter electrodewhereby the adsorption process is made more efficient by not polarisingthe reactor and furthermore power is saved by only polarising thereactor during the relatively short regeneration period.

In a particularly preferred embodiment said nitrogen oxide absorber isregenerated by applying an electrical potential between said nitrogenoxide absorber and said counter electrode in the range from 0 to 1.5 V,preferably from 0.2 to 1.0 V, most preferred from 0.4 to 0.7 V wherebythe potential can be kept as low as possible to save power. In the caseof energetically unfavourable conditions for the reduction, selecting ahigher potential can boost the process.

In still preferred embodiment said regeneration is carried out at anelectrical current density allowing more than 80% regeneration of saidnitrogen oxide absorber after a regeneration time in the range from 5-40s, preferably 5-30 s, most preferred 5-15 s whereby the adsorber isinactive during the regeneration process. Therefore, by minimising theregeneration time and keeping it short compared to the adsorption time,the total reduction rate for the NOx content in the exhaust has can beoptimised.

By changing the length of the adsorption period and the regenerationperiod relative to each other, the process can be adapted to cope withstrongly varying contents of nitrogen oxides in the exhaust gas.

In another preferred embodiment said electrical current density allowingmore than 90% regeneration of said nitrogen oxide absorber after saidregeneration time.

In another preferred embodiment said nitrogen oxide absorber absorbsmore than 60%, preferably in the range 60-80% of the nitrogen oxides ofthe mixutre of nitrogen oxides and oxygen.

In another preferred embodiment said absorption of nitrogen oxides iscarried out to saturation of said nitrogen oxide absorber.

In another preferred embodiment said nitrogen absorber and said workingelectrode are intermixed.

In another preferred embodiment said working electrode is a workingelectrode according to the invention.

Definition of Expressions

The expression electrical current density is intended to designateelectrical current per electrode area, said electrode area typicallybeing the geometrical area of the electrode. In assessment of a measureof an electrode area, adjustment for variations of the microstructureand porosity of the electrode material can be done.

3. BRIEF DESCRIPTION OF THE DRAWINGS

In the following, by way of examples only, the invention is furtherdisclosed with detailed description of preferred embodiments. Referenceis made to the drawings in which

FIG. 1 shows an exemplary cyclic voltametric measurement of a workingelectrode comprising La_(0.82)Sr_(c0.14)Fe_(0.3)Mn_(0.9)O₃ in presenceof nitrogen monooxide, and in presence of oxygen, respectively;

FIG. 2 shows an exemplary cyclic voltametric measurement of a comparisonworking electrode comprising CO₃O₄ presence of nitrogen monooxide, curve1, and in presence of oxygen, curve 2, respectively;

FIG. 3 shows an exempel of cyclic voltametric measurement of a workingelectrode comprising La_(0.85)Sr_(0.15)MnO₃ presence of nitrogenmonooxide, curve B, and in presence of oxygen, curve A, respectively;

FIG. 4 shows five examples of cyclic voltametric measurements of aseries of working electrodes in presence of nitrogen monooxide, saidworking electrodes comprising LSM materials having different degrees ofdoped strontium as cathode;

FIG. 5 shows five examples of cyclic voltametric measurements of aseries of working electrodes in presence of oxygen, said workingelectrodes comprising LSM materials similar to those used for themeasurements shown in FIG. 4;

FIG. 6 shows a cross sectional sketch of an embodiment of anelectrochemical reactor according to the invention;

FIG. 7 shows a cross sectional sketch of an embodiment of anelectrochemical cell for an electrochemical reactor according to theinvention;

FIG. 8 shows a cross sectional sketch of another embodiment of anelectrochemical cell for an electro-chemical reactor according to theinvention; and

FIG. 9 shows a cross sectional sketch of an experimentel electrochemicalset-up for voltametric measurements.

4. DETAILED DESCRIPTION

FIG. 1 shows an exemplary cyclic voltametric measurement of a workingelectrode comprising La_(0.82)Sr_(0.14)Fe_(0.1)Mn_(0.9)O₃ in presence ofnitrogen monooxide, curve B, and in presence of oxygen, curve A,respectively.

The y-axis indicates electric current density in I/μA of the workingelectrode having an electrode area of about 0.01 cm².

The x-axis indicates the potential of the working electrode E in Vversus a standard hydrogen gas electrode of 2.9% H₂ and 3.1% H₂O inargon in equilibrium with a platinum electrode.

An electrochemical cell comprising a working electrode comprisingLa_(0.82)Sr_(0.14)Fe_(0.1)Mn_(0.9)O₃ was prepared according to theprocedure used in Example 1 (see below).

It is seen that at an decreasing potential from about 0.5 V to about−0.1 V, the reaction rate of the reduction of O₂ increases steadily,while the reduction rate for NO is close to zero. For even lowerpotentials, the reaction rate for NO increases very strongly. Theseconditions are not in favour for NO_(x) reduction.

FIG. 2 shows an exemplary cyclic voltametric measurement of a comparisonworking electrode comprising CO₃O₄ in presence of nitrogen monooxide,curve B, and in presence of oxygen, curve A, respectively.

The y-axis indicates electric current density in I/μA of the workingelectrode having an electrode area of about 0.01 cm²

The x-axis indicates the potential of the working 7 electrode E in Vversus a standard hydrogen gas electrode of 3% H₂, and 8% H₂O in argonin equilibrium with a platinum electrode.

FIG. 3 shows an exempel of cyclic voltametric measurement of a workingelectrode comprising La_(0.85)Sr_(0.15)MnO₃ in presence of nitrogenmonooxide, curve B, and in presence of oxygen, curve A, respectively.

The reduction rate for NO (curve B) steadily increases numerically asthe potential is lowered from about 1 V to about 0 V. Note thepolarisation is negative for the working electrode. The reduction ratefor O₂ is very low (close to zero electric current density) from about 0V to about 0.5 V. At lower potentials the reduction rate for O₂increases steadily.

In the range of about 0.9 V to about 0.5 V, the reduction rate for NO ismore than 2 orders of magnitude higher than the reduction rate for O₂.Consequently, the LSM material, here, specificallyLa_(0.85)Sr_(0.15)MnO₃, is very well suited as electrode material forselective reduction of nitrogen oxides in presence of oxygen.

FIG. 4 shows five examples of cyclic voltametric measurements of aseries of working electrodes in presence of nitrogen monooxide, curvesLSM05, LSM15, LSM25, LSM35 and LSM50, said working electrodes comprisingLSM materials having different degrees of doped strontium as cathode.

The designation of the curves LSMy defines used LSM materials of theformula La_((1−x))Sr_(x)MnO₃ wherein y is 100*x, e.g. LSM15 designatesthe LSM matial La_(0.85)Sr_(0.15)MnO₃.

The reduction rate for NO is significantly higher for LSM15 as thecathode material than for any of the other tested LSM materials in therange 0.2 to 0.8 V.

FIG. 5 shows five examples of cyclic voltametric measurements of aseries of working electrodes in presence of oxygen, curves LSM05, LSM15,LSM25, LSM35 and LSM50, said working electrodes comprising LSM materialshaving different degrees of doped strontium as cathode similar as theLSM materials used for the measurements shown in FIG. 4.

It is seen that the reduction rate for O₂increases significantly withincreasing x.

FIG. 6 shows a cross sectional sketch of an embodiment of anelectrochemical reactor according to the invention; The electrochemicalcell comprises an oxygen ion conducting electrolyte 1, here CGO, aselective cathode 2, here an LSM15 material, and an anode 3, hereplatinum.

The electrochemical cell is placed in an gas conduit means 21, 22 for anexhaust gas stream from an engine, here a gas inlet tube 21 and a gasoutlet tube 22. The raw gas stream 11 containing NOx enters the cathodearea 2 of the electrochemical cell, where the NOx is reduced to N, andO₂. The treated gas 12 leaves the cathode area.

The cell is polarised from an external power supply 5 with controlledpotential through the leads 4.

FIG. 7 shows a cross sectional sketch of an embodiment of anelectrochemical cell for an electrochemical reactor according to theinvention.

The electrochemical cell comprises an oxygen ion conducting electrolyte1, a cathode, 2, made from a mixture of cathode catalyst particles 7,here LSM15, and NOx adsorbing particles 8, here BaO particles, and ananode 3, here a platinum electrode.

For illustrative purpose the size of the particles is stronglyexaggerated. In the real cell the particle size was in the range ofabout 0.1 to 10 μm.

FIG. 8 shows a cross sectional sketch of another embodiment of anelectrochemical cell for an electro-chemical reactor according to theinvention.

The electrochemical cell comprises an oxygen ion conducting electrolyte1, a cathode 2, here made from a layer of cathode catalyst material 7,here LSM15, and a porous layer of a NOx adsorbing material 8, heresintered BaO particles, and an anode 3, here a platinum electrode.

FIG. 9 shows a cross sectional sketch of an experimentel electrochemicalset-up for cyclic voltametric measurements.

The electrochemical cell comprises an oxygen ion conducting electrolyte1, a working cathode electrode 2, e.g. made from a layer of cathodecatalyst material as LSM15 and formed in the shape of a cone with anarrow tip for improved positioning of the electrode, said workingcathode electrode further comprising e.g. a porous layer of a NOxadsorbing material 8, here sintered BaO particles, and an anode 3, e.g.a platinum electrode.

The set-up further shows a potentimetric power supply 51, e.g. apotentiostatic power supply supplied by University of Southern Denmark,Odense, supplying electrical currenct through the leads 41, 42.

5. EXAMPLES

Preferred embodiments of the invention are further illustrated byexamples of production of electrochemical cells having workingelectrodes based on LSM materials.

Example 1 “Series of La_(1−x)Sr_(x)MnO₃ Working Electrodes”

“Preparation”

A series of 5 electrochemical cells were produced, each comprising anion selective electrolyte produced by pressing 1 mm thin plates of CGO(cerium oxide doped with 10 atomic-% gadolinium oxide, i.e.Ce_(0.9)Gd_(0.1)O_(1.95), supplied Rhodia Electronics and Catalyst, andsubsequently placing the CGO plates in an electrical furnace sinteringthe plates at a temperature in the range 1400-1550° C. for 2-4 hours.

Working electrodes of the LSM type were provided by depositingLa_(1−x)Sr_(x)MnO₃ onto the exposed upper side of the sintered CGOplates.

LSM materials were prepared by evaporating a solution of thecorresponding metal nitrates, e.g. La(NO₃)₃, Sr(NO₃)₂ and Mn(NO₃)₂stabilised by addition of citric acid. The residue powder was calcinatedat a temperature in the range of 900-1100° C. for 1-3 hours.

A slurry of fine powder of LSM in water was prepared and organic binder,e.g. methylcellulose and other additves, e.g. dispersing agents wereadded to stabilize the slurry.

The slurry was then applied to one side of the sintered CGO plates bypainting or screen printing.

The CGO plates were then sintered further at a temperature in the range1000-1200° C. for 2-4 hours.

Counter electrodes were provided on the other side of the sintered CGOplates by applying platinum paste comprising platinum powder and organicbinder supplied from Engeldhard.

Then CGO plates were then sintered at 8000C for 1 hour.

The preparation of electrochemical cells were repeated with differentLSM materials having values for x in the general formula of 0.05, 0.15,0.25, 0.35, and 0.50.

“Cyclic Voltametric Measurements”

Cyclic voltametric measurements were performed on the producedelectrochemical cells in a N₂ gas containing 2 vol-% NO and in an N₂ gascontaining 10 vol-% O₂. The N₂-gas mixture was supplied by HedeNielsen/Air Liquide, Denmark.

Measurements were performed at temperatures in the range between 300 and500° C. The results are shown in FIGS. 5 and 6 for measurements at 500°C.

At increasing x the reaction rate of the cathodic reduction of O₂increases. At lower x values than 0.25, the reaction rate of O₂ issignificant at potentials below 0.5 V.

It appears that cathodic reduction of NO reaches a maximum at x=0.15.

The experiments show that good selectivity for La_(0.85)Sr_(0.15)O₃between electrochemical reduction of NO and O₂ can be obtained.

Further experiments have shown that similar good selectivity can beobtained for x values in the range of 0.12 to 0.18. Outside this range,the selectivity becomes less good either because of a relatively fasterreduction rate of O₂ and/or a slower reduction rate of NO.

Example 2 “La_(0.85)Sr_(0.15)MnO₃ Based Working Electrode”

An electrochemical cell with a working electrode comprisingLa_(0.85)Sr_(0.15)MnO₃ was prepared as described in Example 1. The cellwas tested at a temperature of 300° C. in a flowing N₂ gas containing1000 ppm NO and 10 vol.-% O₂. The cell was polarised with 0.5 volts.

Because some NO₂ is formed in the mixture of NO and O₂, the contents ofNO and NO₂ were measured in the exhaust gas of the electrochemical cellby mass spectrometry analysis using a Varian mass spectrometer. Thereduction rate of NO was measure in the range of 40 to 80% depending onthe gas flow rate through the cell, said reduction rate being based onthe combined content of NO and NO₂ measured.

Example 3 “La_(0.85)Sr_(0.15)MnO₃, and BaO Based Working Electrode”

An electrochemical cell with a working electrode comprising a mixture of50 weight-% La_(0.85)Sr_(0.15)MnO₃ and 50 weight-% BaO supplied fromMerck was prepared as described in Example 1. During the preparation,the working electrodes were activated by addition of platinum. Platinumwas added by impregnating a solution of PtCl₄ in 0.1 N hydrochloric acidinto the working electrode material. Then the working electrode weredried and heated to a temperature of 600° C. BaO functions as anabsorber of nitrogen oxides. Pt functions as an auxiliary catalyst forthe NOx adsorption reactions.

Depending on the exact composition of the exhaust gas several possiblereaction can take place, e.g.2 NO+1.50₂+BaO->(Pt) Ba(NO₃) ₂

The electrochemical cells were tested at a temperature of 300° C. in aN₂ gas containing 1000 ppm NO and 10% O₂. The cells were run for 2 minwithout polarisation of the working electrode allowing NO to becomeabsorbed into the working electrode BaO material. Then the workingelectrode was polarised with 0.5 volts for 20 seconds.

The content of NO and NO₂ were measured in the exhaust gas of theelectrochemical cell. The reduction rate of NO was between 60 and 90%depending on the gas flow rate through the cell, said reduction ratebeing based on both the measured contents of NO and NO₂.

Example 4 “Energy Consumption—Calculation”

In a typical automobile with turbo charged diesel engine of 2 ldisplacement, driving at constant speed of 120 km/h, NO_(X) in exhaustgas is typically 750 ppm.

Under these conditions, the engine will deliver about 20-25 kW. Theexhaust gas flow will be about 80 l/s. The temperature will be about300° C.

For simplicity NO_(x) is calculated as NO, since in a diesel engine, theNO content is more than about 90 vol.-% of the total NO_(x). The NOcontent is 80*750 ppm=0.06 l/s.

The number of moles of NO is n=0.06/0.082/575=0.00127 mol/s.

Multiplying with Faradays constant and multiplying with 2 for the numberof electrons in the reaction provides the demand for current:I=0.00127*96500*2=246 A

With a current efficiency of 60% and a potential of 0.5 volts, thisprovides a power demand ofP=216*0.5/80*100=205 W

This corresponds to 0.8% of the engine's power.

1-23. (Cancelled)
 24. A method of reducing nitrogen oxides in a mixtureof nitrogen oxides and oxygen, by use of an electrochemical reactorcomprising a working electrode for reducing nitrogen oxides to nitrogenand oxygen, a counter electrode, and an ion-selective electrolyte; theprocesses taking place at the electrodes being substantially as follows:at the cathode2NO_(x)+2xe⁻->N₂+2xO₂  (a)O₂+4e⁻->2O²⁻  (b) at the anode2O²⁻->O₂+4e⁻  (c) said cathode electrode processes (a) and (b) beingcarried out at a potential between −1500 my and +1500 mV between saidworking electrode and said counter electrode, and at a temperaturewithin a range of 200 to 500° C.; and said working electrode comprisingan electric conductive ceramic material of lanthanum manganite (LaMnO₃),lanthanum chromite (LaCrO₃), lanthanum ferrite (LaFeO₃), lanthanumcobaltite (LaCoO₃) or lanthanum nickel oxide (LaNiO₃); said materialbeing doped with one or more of metals selected from the groupconsisting of Sr, Ca, Ba, Eu, Fe, Co and Ni in an effective amount toachieve a faster rate of reduction of nitrogen oxides than the rate ofreduction of oxygen at the selected potential and temperature.
 25. Amethod according to claim 24, wherein the potential of the workingelectrode is between −200 mV and 800 mV, measured versus a hydrogenelectrode of 8% H₂O and 3% H₂ in Ar.
 26. A method according to claim 24or 25, wherein the working electrode is La_(1−x)Sr_(x)MnO₃ with x beingin the range from 0.12 to 0.18.
 27. A method according to claim 24,wherein the mixture of nitrogen oxides and oxygen is concentrated withan absorber capable of absorbing nitrogen oxides and selected from thegroup consisting of Na₂O, K₂O, MgO, CaO, SrO, and BaO and that thenitrogen oxides absorbed in the absorber are caused to react with theworking electrode.
 28. A method according to claim 27, wherein theabsorber comprises a working electrode and a counter electrode forcausing the absorbed nitrogen oxides to react with the working electrodeof the electrochemical reactor by establishing an electric potentialbetween said electrodes.
 29. A method according to claim 28, wherein thenitrogen oxides are absorbed without applying any electrical potentialbetween the working electrode of the absorbing material and the counterelectrode of the absorbing material.
 30. A method according to claim 27,wherein nitrogen oxides are reduced at the same time as the absorber isregenerated.
 31. A method according to claim 28, wherein said absorberis regenerated by applying an electrical potential between the workingelectrode of the absorber and the counter electrode of the absorber inthe range of from 0 to 1.5V.
 32. A method according to claim 30 or 31,wherein said regeneration is carried out at an electrical currentdensity causing more than 80% regeneration of said absorber after aregeneration time in the range from 5 to 40 s.
 33. A method according toclaim 32, wherein said electrical current density causes more than 90%regeneration of said nitrogen oxide absorber after said regenerationtime.
 34. A method according to claim 27, wherein said absorber absorbsmore than 60%, preferably in the range 60-80% of the nitrogen oxides ofthe mixture of nitrogen oxides and oxygen.
 35. A method according toclaim 27, wherein said absorption of nitrogen oxides is continued untilthe absorber is saturated.
 36. A method according to claim 27, whereinsaid absorber and said working electrode are intermixed.
 37. Anelectrochemical reactor for reducing nitrogen oxides in a mixture ofnitrogen oxides and oxygen, comprising a working electrode for reducingnitrogen oxides to nitrogen and oxygen, a counter electrode, anion-selective electrolyte where said working electrode comprises anelectric conductive ceramic material of lanthanum manganite (LaMnO₃),lanthanum chromite (LaCrO₃), lanthanum ferrite (LaFeO₃), lanthanumcobaltite (LaCoO₃) or lanthanum nickel oxide (LaNiO₃); said materialbeing doped with one or more of metals selected from the groupconsisting of Sr, Ca, Ba, Eu, Fe, Co and Ni in an effective amount toachieve a faster rate of reduction of nitrogen oxides than the rate ofreduction of oxygen, wherein the reactor further comprises means formaintaining a potential between −1500 mV and +1500 mV between saidworking electrode and said counter electrode and means for maintaining atemperature within a range of 200 to 500° C.
 38. A reactor according toclaim 37 which further comprises a nitrogen oxide absorber.
 39. Areactor according to claim 37 or 38, wherein said nitrogen oxideabsorber comprises a material or a combination of materials selectedfrom the group consisting of Na₂O, K₂O, MgO, CaO, SrO and BaO.
 40. Amethod according to claim 28, wherein the working electrode of theabsorber comprises an electric conductive ceramic material of lanthanummanganite (LaMnO₃), lanthanum chromite (LaCrO₃), lanthanum ferrite(LaFeO₃), lanthanum cobaltite (LaCoO₃) or lanthanum nickel oxide(LaNiO₃); said material being doped with one or more of metals selectedfrom the group consisting of Sr, Ca, Ba, Eu, Fe, Co and Ni in aneffective amount to achieve a faster rate of reduction of nitrogenoxides than the rate of reduction of oxygen at the selected potentialand temperature.